This invention relates generally to oscillators. Specifically, the present invention relates to oscillators having an output signal which oscillates at a frequency selected from a discrete number of possible frequencies. More specifically, the present invention relates to oscillators which maintain a predetermined frequency shift between the possible frequencies as the frequencies drift in response to temperature changes.
The oscillator of the present invention is useful in frequency shift keying (FSK) and other applications needing precise oscillator frequencies and highly reliable oscillator circuits. As is well known in the art, crystal controlled circuits may partially achieve these needs by addressing the precision criteria. However, the reliability criteria suffers when circuits employ unreliable parts or an excessive number of parts. Additionally, circuits which employ too many parts are undesirable because they tend to cost more than circuits which use fewer parts.
One prior art crystal controlled circuit provides a separate crystal for each discrete frequency at which the circuit oscillates. Each crystal is contained within its own sub-oscillator and the sub-oscillators are multiplexed together to generate the output signal. However, such a multiple crystal circuit fails to meet design needs in two ways.
First, the reliability of the oscillator suffers because the oscillator requires excess parts in the form of the multiple sub-oscillator circuits and the multiplexer. Further, the crystal is a relatively unreliable and costly part whose duplication disproportionately hurts the overall circuit reliability and cost.
Second, the multiple crystals technique harms the overall shift accuracy parameter of the oscillator. Shift accuracy characterizes an important parameter in multiple discrete frequency oscillators, like the circuit contemplated in the present invention. These oscillators exhibit shifts or differences in frequency between the discrete frequencies. Switching the frequency of oscillation from one frequency to another produces this frequency shift. Furthermore, in many applications this shift accuracy is more important than the accuracy of the absolute values of the particular frequencies.
As is well known in the art, oscillators which utilize crystals tend to demonstrate some drift in frequency as the temperature of the circuit changes. The frequency drift versus temperature characteristic will not be identical among the various crystals and sub-oscillators in a circuit. This inequality of performance causes the crystals and sub-oscillators to drift relative to each other over a range of temperature. Additionally, since several crystals and corresponding sub-oscillator components cannot be placed in the exact same physical locations in a circuit, the several crystals and sub-oscillator components experience slightly different temperatures. Both these factors combine to cause various sub-oscillators to independently drift in frequency as a function of temperature. Thus, even a circuit adjusted to precisely generate a predetermined shift at one temperature fails to meet design needs when temperature changes cause one frequency to drift too far relative to other frequencies.
The prior art teaches the use of various ovens and temperature compensation schemes to prevent crystal controlled oscillators from drifting as a result of temperature changes. These techniques improve frequency shift accuracy by limiting the drifting over temperature of each of the discrete frequencies. However, these techniques require considerable design effort involving computer analysis and much development testing. Furthermore, these techniques typically use low reliability parts and an excessive number of parts. Therefore, these techniques fail to meet design needs because they yield a poor overall reliability.
An improvement over the multiple crystal circuits utilizes only one crystal and a varactor diode to generate a plurality of output frequencies. Changing a reactance of a circuit coupled to a crystal "pulls" the crystal to a different frequency. Varying the reverse bias voltage across a varactor diode causes the varactor to exhibit a changing capacitance. Thus, the varactor's capacitance change causes the reactance change which tunes the circuit's frequency of oscillation.
The varactor technique improves on the reliability attained from the multiple crystal circuit. Since varactor techniques use the same parts to generate oscillations at different frequencies, the number of parts in the circuit decreases. This reduction in parts tends to increase the reliability. Additionally, such a technique reduces the number of crystals, which are relatively unreliable parts, and thus improves the overall circuit reliability.
The shift accuracy parameter may also slightly improve by using a varactor technique rather than a multiple crystal technique. Specifically, the varactor technique eliminates problems associated with different sub-oscillator circuits having different temperature stability characteristics and different sub-oscillator components experiencing different temperatures within the circuit.
However, varactor techniques still fail to meet design needs. Reliability suffers because varactors require the use of additional parts. Specifically, the frequency accuracy of an oscillation depends on the accuracy of the capacitance exhibited by the varactor. The varactor capacitance accuracy further depends on the accuracy of the reverse bias voltage applied to the varactor. Thus, generating a plurality of precise frequencies requires the application of a corresponding plurality of precise voltages to a varactor. The precise voltage requirement necessitates the addition of accurate voltage regulators and accurate voltage switching devices to the oscillator circuit. Accordingly, the addition of these components harms reliability.
Additionally, varactor techniques tend to decrease the frequency accuracy a particular frequency of oscillation maintains over temperature. Frequency accuracy suffers because semiconductor devices, such as varactors and voltage regulators, determine the frequency. As is well known in the art, semiconductor devices tend to demonstrate a relatively poor temperature stability when compared with the stability of certain passive devices, such as various types of capacitors. For example, a typical temperature coefficient of capacitance for a varactor is significantly worse than that of certain types of capacitors. Thus, the capacitance of a varactor experiences a greater drift resulting from a given temperature change than the capacitance drift experienced by such capacitors. Accordingly, a frequency from an oscillator using a varactor drifts more with temperature and exhibits a worse frequency accuracy than a frequency of an oscillator using such a capacitor.
Varactor techniques also tend to harm the related parameter of shift accuracy. If each of the plurality of frequencies generated by the oscillator tended to drift equally in response to temperature changes, then shift accuracy over temperature would not suffer. However, the temperature coefficient of capacitance of a varactor significantly varies as a function of the reverse bias voltage applied to the varactor. Since varactor techniques determine frequency values for each of the plurality of frequencies by applying different reverse bias voltages to the varactor, different temperature coefficients characterize each of the plurality of frequencies. Therefore, some frequencies drift more than others in response to a given temperature change. Varactor techniques cannot maintain over temperature a shift which has been precisely adjusted at any one given temperature.